The invention relates to a pipe system for receiving and transporting lime sludge and mud from a filter medium in a continuously operating filter in a causticizing process, in which white liquor is separated from the lime sludge.
The green liquor entering the causticizing process consists fundamentally of sodium carbonate. In the process, the green liquor is made to react with burnt lime, calcium carbonate (lime sludge) and sodium hydroxide (white liquor) being formed. The white liquor is filtered off in tube filters or disc filters, the latter in particular usually being of the pressurized type, after which the white liquor is reused in the sulphate process for cooking pulp wood chips. The lime sludge obtained from filtration is burnt so as to form burnt lime again.
Modern effective filtration arrangements for white liquor filtration produce lime sludge/filter cakes with relatively high dry contents but in which the lime sludge cakes, after they have been removed from the filter medium, constitute a material with highly adhesive properties. The material of the lime sludge cakes therefore has a tendency to adhere to the inner walls in the chute(s) via which it is transported away from the filter medium. This can give rise to acute operational stoppages or stoppages for cleaning on account of clogging, which reduces the capacity of the filtration arrangements and can cause problems and lead to other disadvantages.
The cleaning of ducts through which adhesive substances are transported can be effected by, for example, manual cleaning or automatic cleaning. Manual cleaning has many obvious drawbacks such as operational stoppages or partial operational stoppages and a poor working environment. Automatic cleaning can be performed using well-known methods such as, for example, with brushes, beating tools, ultrasound, steam-jetting, and liquid application where this is possible.
In continuously operating filtration arrangements for liquid suspensions such as, for example, those described in SE-C-463771 (=U.S. Pat. No. 4,929,355), a filter cake in the form of lime sludge is formed on a filter medium. This lime sludge is scraped off by doctors (knives) and drops down into receiving chutes, in which further dilution by dilution liquid is essential for it to be possible to transport the lime sludge onward. In said SE-C-463771, a feed-out screw is shown directly after the feed-out chutes in one embodiment, which screw feeds onward to a feed-out valve arrangement. In another embodiment shown, this arrangement is illustrated in the form of a level-regulated storage tank, in which there is a mechanical agitator. Some embodiments of this process comprise a sloping collecting pipe after the feed-out chutes, which collecting pipe leads to the storage tank. In order to avoid sedimentation in the sloping pipe, the lime sludge slurry is circulated from the bottom of the storage tank to the highest point of the sloping collecting pipe.
A corresponding solution with a storage tank and mechanical mixer is disclosed in U.S. Pat. No. 5,151,176. In other applications with tube filters as well (see U.S. Pat. No. 4,264,445), use is made of a storage tank and a mechanical mixer in order for it to be possible to deal with the filter cake taken out of a pressurized tube filter. The bulky storage tank and the mechanical agitator arranged therein were considered necessary in order for it to be possible to keep the lime sludge well mixed with added dilution liquid and to prevent sedimentation of the lime sludge.
The known art has led to handling systems for the lime sludge being unnecessarily expensive and space-consuming. Conventionally employed treatment vessels with mechanical agitators for receiving the lime sludge resulted in considerable costs. In the case of application in a white liquor filter with a relatively low capacity of 2,500 m3/24 hrs, and a vessel with dimensions of 1900xc3x972800 (diameterxc3x97height), the cost can be in the order of SEK 800,000 (approximately US$ 100,000). In a white liquor filter with a higher capacity of 8,000 m3/24 hrs, and a vessel with dimensions of 2800xc3x973300 (diameterxc3x97height), the cost can be in the order of SEK 1,500,000 (approximately US$ 200,000).
Another disadvantage is that the mechanical agitators, in particular when applied in pressurized white liquor filters, have to be provided with packing boxes, which often lead to leakage problems.
The space requirement for storage tanks also limits the possibilities of structural alterations in existing installations where available space does not always allow the storage tank to be positioned adjacent to the filter, or results in the filter and the storage tank having to be located on different floors in the installation. In most cases, the storage tanks used in white liquor filtration have been virtually the same size as the white liquor filter itself, which is in itself very space-consuming.
One object of the invention is to provide a system for receiving and transporting lime sludge from a filter medium in a continuously operating pressurized filter in a causticizing process, in which white liquor is separated from the lime sludge, and in which the system can be minimized in terms of size and cost at the same time as the conventionally used mechanical agitators can be dispensed with entirely, notwithstanding the strong tendency of the lime sludge to adhere to the walls in the handling system.
Another object is to obtain a system in which the dry content can be kept high in the system, which high dry content has not been considered possible in the large storage tanks as the lime sludge is to be kept in a uniform solution throughout the storage tank. The total quantities to be handled are then smaller, which results in considerable savings on pipeline systems and associated equipment.
Another object is to permit structural alterations in existing installations in which the available space is limited, it being possible then to arrange high-performance filter equipment in a small available space.
In a preferred embodiment, the risk of sedimentation in the system is reduced by using forced recirculation which affords an increased speed throughout the volume of the lime sludge mixture received. In an advantageous embodiment, a modified recirculation principle can be used in order to keep all the receiving chutes free from accumulations of lime sludge without having to dilute the lime sludge further with dilution liquid. In this embodiment, the storage tank can be minimized or eliminated completely.
Further characteristics, aspects and advantages of the invention emerge from the patent claims below and from the following description of a number of possible embodiments.